DE102010027294A1 - Conductive and hydrophilic surface modification of a bipolar plate for fuel cells - Google Patents

Abstract

A fuel cell includes a bipolar plate having a conductive and hydrophilic surface layer disposed on at least a portion of its exterior. The surface layer comprises a conductive carbon material having a hydrophilic organic group covalently attached to its surface. A method of manufacturing a bipolar plate and a fuel cell is also disclosed.

Description

Technical area

The technical field to which the disclosure generally relates includes bipolar plates for fuel cells.

background

A fuel cell usually consists of a series of membrane electrode assemblies and bipolar plates stacked in an alternating manner. The membrane electrode assembly is typically made of an ion conducting membrane disposed between anode and cathode portions respectively disposed on opposite sides of the membrane. A bipolar plate is a plate-like electrical conductor having a plurality of channels for fluid passage. Reactive gases that supply a fuel cell flow through these channels to reach the anode and cathode sections where electrochemical reactions of the gases take place and from which electricity is generated. The electricity generated by the electrochemical reactions is collected and passed through the bipolar plate to an external circuit. Thus, the bipolar plate must have high electrical conductivity or low contact resistance to reduce energy loss and heat generation.

In the case of a hydrogen fuel cell, the water balance is one of the biggest challenges. Water is constantly generated in a hydrogen fuel cell and the ion-conducting membrane must maintain a certain degree of hydration. When a hydrogen fuel cell is operated at a low current density, e.g. At 0.2 A / cm ² , there is insufficient gas flow to remove the water generated by the electrochemical reaction in the cathode section. In the fluid passages, water droplets may form and block the supply of the electrode with a reactive gas. Without the supply of a reactive gas, the blocked portion of the fuel cell does not generate electricity. The performance of the fuel cell will degrade due to the non-homogeneous current distribution. Such a phenomenon is known as low power stability (LPS).

Conventional hydrophilic coatings or treatments on a bipolar plate can improve the water balance but usually negatively affect the electrical contact resistance. Conventional hydrophilic coatings and treatments can also result in an increase in water leachable pollutants and electrochemical degradation of the bipolar plate, electrodes, and membranes.

Summary of exemplary embodiments of the invention

One embodiment includes a bipolar plate for a fuel cell having a conductive and hydrophilic surface layer disposed on at least a portion of its exterior. The surface layer comprises a conductive carbon material having a hydrophilic organic group covalently attached to its surface.

Another embodiment includes a method of manufacturing a bipolar plate, comprising providing a bipolar plate for a fuel cell with a carbon material mounted on at least a part of its outer region, at least bringing the carbon material into contact with an organic molecule a hydrophilic group and causing the organic molecule to react with the carbon material so that the hydrophilic group is covalently attached to the carbon material.

Another embodiment includes a method comprising providing a carbon material and a bipolar plate for a fuel cell, covalently attaching a hydrophilic organic group on the carbon material by a chemical reaction, and depositing the carbon material on at least a part of the exterior of the bipolar plate to form a conductive and hydrophilic surface layer. Using the bipolar plate, a fuel cell stack can be assembled.

Further exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be assumed that the detailed Description and specific examples, while disclosing exemplary embodiments of the invention, are for the purpose of illustration only and are not intended to limit the scope of the invention

Brief description of the drawings

Exemplary embodiments of the present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

1 Fig. 3 is a schematic illustration of an exemplary chemical change of a carbon material surface layer.

2 Figure 3 is a schematic illustration of an exemplary ionic adsorption process for depositing a hydrophilic and conductive carbon material on the surface of a bipolar plate.

3 Figure 3 is a schematic illustration of an exemplary chemical change of the exterior of a bipolar plate.

4 Figure 3 is a schematic illustration of an exemplary chemical modification of a bipolar plate having a carbon material surface layer.

5 is a schematic drawing of an exemplary fuel cell stack in an unassembled perspective view.

Detailed description of exemplary embodiments

The following description of embodiment (s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

Various chemical reactions can be used to alter the surface of a carbon material and make it hydrophilic. An exemplary reaction between a carbon material and an aromatic diazonium salt is disclosed in U.S. Pat 1 illustrated. This example allows an organic diazonium salt 11 represented by the formula X ^- N ₂ ⁺ -Ar -SO ₃ H, with a carbon material 10 reacts to a changed carbon material 12 with a hydrophilic aromatic sulfonate (-Ar-SO ₃ ) group covalently attached to the carbon material surface. As a product of such a reaction, nitrogen gas, N ₂ , is released. "Ar" in the diazonium salt is an aromatic radical, "N ⁺ _2" is a Diazoniumradikal and "X ^-" is an anion. Similar reactions between a diazonium salt and a carbon material are described in U.S. Pat U.S. Patent Nos. 5,554,739 and 5,922,118 which are incorporated herein by reference. Other chemical reactions between a carbon material and a molecule comprising a hydrophilic group may be employed in a similar manner to covalently attach a hydrophilic group to a carbon material.

All organic molecules could be used to react with a carbon material comprising a hydrophilic group HG represented by the chemical formula X ^- N ₂ ⁺ -Ar-HG. Examples of the anionic group, X ^- , may include sulfate, carbonate, nitrate, chloride, bromide, iodide, fluoride, phosphate, borate, chlorate, hydroxide and silicate. Examples of the aromatic radical, Ar, may include a radical of benzene, naphthalene, alkylbenzene, biphenyl, triphenyl, phenol, pyridine, anthracene, pyrene, phenyl ether, phenyl ester and any other of their corresponding derivatives. A hydrophilic group, -HG, is an organic radical with a strong polarity and is capable of rendering the surface of a carbon material hydrophilic when covalently attached to the carbon surface. The hydrophilic group may be an anionic, cationic or nonionic radical. The ionic character of the hydrophilic groups attached to carbon increases the robustness against contamination, ie the surface remains longer hydrophilic in the presence of air hydrocarbons. Examples of anionic hydrophilic groups include sulfonic acid, carboxylic acid, phosphonic acid, boronic acid and their corresponding salts. Examples of nonionic hydrophilic groups may include a radical of alcohol, ketone, ethoxyl, polyethylene oxide, methoxyl, amide and urea. Examples of cationic hydrophilic groups can include a radical of primary amine, secondary amine, tertiary amine, quaternary amine, pyridinium and phosphonium groups.

The organic molecule, X ^- N ₂ ⁺ -Ar-HG can be prepared in situ by the combination of an aromatic amine comprising a hydrophilic group, -HG, with a nitrite salt, nitrous acid, nitrogen dioxide or a mixture of nitric oxide and nitrogen dioxide. Examples of aromatic amines include sulphanilic acid, 4-aminobenzoic acid, 4-aminosalicylic acid, 7-amino-4-hydroxy-2-naphthalenesulfonic acid, aminophenylboronic acid, aminophenylphosphonic acid, 4-aminophthalic acid, 2-amino-1-naphthalenesulfonic acid, 5-amino-2-naphthalenesulfonic acid , Metanilic acid, N- (4-aminobenzoyl) -B-alanine, N- (4-aminobenzoyl) -L-glutamic acid, p-aminohippuric acid, 2-naphthylamine-1-sulfonic acid (Tobias acid) and 1-amino-4- (trialkylamino ) benzene. If a nitrite salt, such as. As sodium nitrite, potassium nitride or magnesium nitride is used, an acid can be further included to facilitate the formation of the diazonium salt molecule. Any organic or inorganic acid may be used, including e.g. For example, formic acid, acetic acid, hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid and toluenesulfonic acid. Several exemplary reactions are shown in the following reaction schemes (1-3):

The carbon material may include any electrically conductive form of carbon or carbon composite. Examples of carbon material may include graphite, carbon black, amorphous carbon, carbon fiber, carbon nanotubes, and carbon composite.

The organic diazonium molecule can react with carbon material in a wide range of conditions. The reaction may be carried out in a temperature range from 0 ° C to about 100 ° C and at a pH between 1 and 9. The reaction can be carried out in an aqueous solution, in a polar organic solvent or in a solvent mixture. The reaction can easily take place in an aqueous solution or in the presence of an acid. Depending on the ratio between the amount of organic molecules and the surface area of the carbon material, various amounts of the hydrophilic groups may be covalently attached to the surface of the carbon material. The amount of covalently attached hydrophilic groups on the carbon material may be between about 0.01 to about 5 millimoles per square meter (mmol / m ² ) or 0.1 to 4 mmol / m ² . The water contact angle of the altered carbon material surface layer is typically below about 40 degrees or below 20 degrees.

The altered carbon material with a hydrophilic group, HG, covalently attached to its surface, exhibits hydrophilic properties and good electrical conductivity. Unlike a conventional coating using inorganic oxides or hydrophilic resins, the covalently attached hydrophilic group does not leach into water during fuel cell operation. The hydrophilic effect is very durable and in a wide range of fuel cell operating conditions electrochemically stable. The electrical contact resistance of the altered carbon surface layer is typically similar to that of the carbon surface layer without change. Although applicant does not wish to be bound by or bound by any particular theory, it is believed that the strong polarity of the hydrophilic group at a molecular thickness on the surface of a carbon material does not adversely affect the electrical conduction on a contact interface. When an ionic hydrophilic group is attached to the carbon material surface, an ionic conduction mechanism is also possible. Some of the hydrophilic groups, such as. B. benzenesulfonic acid, benzenesulfonic acid, benzenecarboxylic acid, phthalic acid, terephthalic acid, isophthalic acid, their corresponding salts and derivatives show high electrochemical stability and thus withstand harsh operating conditions in a sophisticated fuel cell design.

A modified carbon material may be deposited on a bipolar plate for fuel cells using various deposition methods, including, but not limited to, ionic adsorption, chemical vapor deposition, physical vapor deposition, atomic layer coating, spray coating, plasma deposition, dip coating, Drawndown coating, printing, electrochemical coating and heat spraying. An exemplary ionic adsorption process is described in US Pat 2 illustrated. A bipolar plate 13 is first using a solution with a cationic species 14 brought into contact. The cationic species is typically a molecule or a resin comprising a cation that can be adsorbed or otherwise bonded to the surface of the carbon material. Exemplary cationic species may include polyamines, quaternary ammonium salts, polyethylenimine, diethylaminoethyl (meth) acrylate polymers and copolymers, triethylaminoethyl (meth) acrylate polymers and copolymers, dimethylaminoethyl (meth) acrylate polymers and copolymers, trimethylaminoethyl (meth) acrylate Polymers and copolymers, copolymer of acrylamide and trimethylaminoethyl methacrylate methylsulfate, organic phosphonium salt, polyallylamine hydrochloride, polydiallyldimethylammonium chloride. It becomes the bipolar plate 15 with adsorbed cationic species and this can be rinsed in water to remove any unbound ionic species. The bipolar plate 15 adsorbed species are then contacted with a dispersion containing the altered carbon material 12 with an anionic hydrophilic sulfonate group, -Ar-SO ₃ H. The strong ionic attraction between the adsorbed cation and the anionic sulfonate group on the carbon material causes the carbon material 12 deposited on the bipolar plate as a surface layer. The ionic adsorption process can be repeated numerous times to obtain multilayers of the carbon material on the outside of the bipolar plate. A bipolar plate 17 with two layers of the modified carbon surface layer may, for. B. prepared as in 2 illustrates when the ionic adsorption is carried out twice. A particle of carbon material can be deposited on a bipolar plate as in 2 to form a microporous two-dimensional or three-dimensional layer (s). The microporous structure is characterized by the micro- or nanoscale pores or voids between carbon particles. Some of the pores or cavities may be interconnected. Such a microporous structure can effectively spread a drop of water before it blocks a gas passage and increase the water wicking action of the surface of a bipolar plate for improved water balance in a fuel cell. The carbon particle may have an average particle size of less than about 40 microns or 325 mesh sizes. Accordingly, a particle size between about 1 nanometer and 40 micrometers can be used. Examples of particulate carbon materials may include carbon black, carbon nanotubes, milled graphite, fullerene, synthetic carbon microspheres, and the like. The altered carbon particles may also be applied to the exterior of a bipolar plate by dip coating, spray coating, printing or the like. A resin binder may be used in combination with the altered carbon particle to form a durable coating layer. In yet another embodiment, in addition to the cationic polymer used in the ionic adsorption coating process, a metal oxide such as e.g. B., but not exclusively, TiO ₂ or ZnO be used. That is, the metal oxide can be deposited on the bipolar plate, and then the bipolar plate becomes 15 with the metal oxide thereon with a dispersion comprising the altered carbon material 12 , brought in contact. The hydrophilic carbon is ionically attached to the positively charged hydroxyl groups of the metal oxides (low pH, 3 to 5). These metal oxides are hydrolytically more stable than the polymer cations.

The substrate of the bipolar plate can be constructed using all electrically conductive and mechanically robust materials. Examples of bipolar plate substrates may include graphite plates, carbon fiber reinforced composite, carbon fiber reinforced carbon composite, stainless steel alloys, titanium, titanium alloys, copper alloys, aluminum, aluminum alloys, and the like. When a metal or metal alloy as the substrate of a bipolar plate a corrosion resistant and conductive coating or treatment can be applied to the surface of the bipolar plate before a layer of carbon material is deposited. The corrosion resistant coating or treatment may include precious metal (such as gold, platinum, ruthenium, rhodium, palladium, indium, and osmium), conductive metal oxides, metal nitrides, metal oxynitrides, and carbon. A bipolar metal plate may also be passivated by anodization chemically or electrochemically. The corrosion resistant coating or treatment may be applied by chemical vapor deposition, physical vapor deposition, atomic layer coating, spray coating, plasma deposition, dip coating, electrochemical coating and heat spraying techniques.

A bipolar plate can be made using a graphite or carbon composite material. The gas flow channel and other mechanical features may be formed on the plate or mechanically generated. As in 3 Illustrated is a bipolar carbon plate 20 with a solution comprising a diazonium salt represented by the chemical formula HG-Ar-N ₂ ⁺ X ^- brought into contact. HG is a hydrophilic group, Ar is an aromatic group, -N2 ⁺ is a diazonium group and X ^- is an anion. The diazonium salt can be prepared or generated in situ as described above. The diazonium salt readily reacts with the carbon-based surface of the bipolar plate, resulting in covalent attachment of the organic hydrophilic group, -Ar-HG, on the carbon surface of the bipolar plate. Thus, a bipolar plate 21 with covalently attached organic hydrophilic groups. Various anionic, cationic and nonionic hydrophilic groups can be covalently attached to the bipolar plate using this method.

With reference to the 4 For example, a bipolar plate can be made by first applying a carbon layer 23 on a metal, a carbon or a composite insulation board 22 is attached. The carbon layer 23 can be determined using all known storage methods such. For example, physical vapor deposition, chemical vapor deposition, coating, plasma deposition, atomic layer deposition, and heat spray processes are deposited. Like in the 4 the bipolar plate having a carbon layer is then shown with a diazonium salt, HG-Ar-N2 ⁺ X ^-, brought into contact, as described above. The chemical reaction between the diazonium salt and the carbon layer leads to a covalent attachment of the organic group -Ar-HG on the carbon layer of the bipolar plate 24 , The carbon layer may be a solid carbon layer or a microporous carbon layer, depending on the deposition method used and the choice of carbon material.

A fuel cell can be manufactured using the bipolar plate described above. With reference to the 5 is an exemplary fuel cell with a membrane electrode assembly (MEA) 39 which is incorporated therein together with other fuel cell stack components are shown in a non-assembled form. The MEA 39 comprises an ion-conducting electrolyte membrane 35 between an anode containing catalysts 40 and a catalyst-containing cathode 34 is arranged. The in the 5 shown fuel cell includes stainless steel end plates 33 . 43 , the bipolar plate 37 with gas flow channels 36 to facilitate the gas supply and distribution, and the membrane electrode assembly 39 , The stacking unit 38 which is a bipolar plate 37 and an MEA 39 can be repeated numerous times for a higher energy efficiency. The fuel cell includes reactant gases, one of which is a fuel (such as hydrogen) from the fuel inlet 42 and another is an oxidizing gas coming from the inlet 31 is delivered. The corresponding outlets for the fuel gas 32 and the oxidizing gas 41 are also provided. The fuel gas may be a hydrogen gas and the oxidizing gas may be air or hydrogen. The gases provided to the fuel cell diffuse through respective bipolar plates 37 to the corresponding anodes and cathodes of the MEA 39 for electrochemical reactions and for the generation of energy. A gas diffusion medium (typically made of carbon cloth or carbon paper, not shown in the drawing) may also be included between the bipolar plate and the MEA.

example

10 grams of carbon black powder, Vulcan XC-72, obtained from Cabot Corporation, was mixed with 250 ml of 0.05 M nitric acid (HNO ₃ ), 15 ml of isopropyl alcohol (as wetting agent) and 30 millimoles of sulphanilic acid (purchased from Aldrich). The mixture was stirred and kept at about 0 ° C. A solution of 33 millimoles of sodium nitrite (NaNO ₂ ) dissolved in 30 ml of water was added dropwise to the above mixture with stirring. The mixture was stirred for 2 hours at about 0 ° C before 150 millimoles of formic acid was added dropwise to the mixture. The mixture was allowed to gradually warm to room temperature overnight and then heated to about 70 ° C for 1 hour. The carbon black surface is thus changed by a covalently attached benzenesulfonic acid group, as shown in the following reaction scheme:

The altered carbon in the mixture is separated by centrifugation and purified in water by an ultrasonic centrifuging-decanting method. The cleaned carbon material is dried in vacuum.

By a physical vapor deposition method, a ferritic (e.g., 430) or austenitic (e.g., 316L) bipolar stainless steel plate having gas flow channels formed by an embossing process is deposited with a gold surface layer.

An ionic adsorption process was then used to form a hydrophilic and conductive carbon surface layer on the bipolar plate. A cationic layer was first adsorbed on the bipolar plate by immersing the bipolar plate in an aqueous solution of poly (trimethylaminoethyl methacrylate-co-acrylamide) methyl sulfate. The bipolar plate was then rinsed in deionized water before being immersed in a solution of the modified Vulcan XC-72 carbon black as described above. Due to the strong ionic interaction between the covalently attached anionic sulfonic acid group and the adsorbed cationic polymer, the altered carbon black formed a highly adsorbed carbon surface layer. The bipolar plate was then rinsed in water to remove any salt residues. The above-mentioned ionic adsorption process was performed 2, 4 and 8 times (referred to herein as 2-dip, 4-dip and 8-dip) individually on 3 separate bipolar plates before the bipolar plates were dried. The anode side of the bipolar plate was pressed at various pressures against a Teflon-coated carbon paper gas diffusion layer to measure the contact resistance (also called interfacial contact resistance). The results are shown in the following diagram. A contact resistance of about 20 milliohm cm ² (mohm cm ² ) at about 150 psi can be obtained. As shown in the diagram below, the gold-coated bipolar plate without a carbon surface layer has a very similar contact resistance to similar bipolar plates having a deposited modified carbon surface layer. In addition, the bipolar plate having a conductive and hydrophilic carbon surface layer exhibits a water contact angle of initially 10 degrees or less and less than about 20 degrees after 45 days of soaking in 80 ° C hot water, exhibiting a lasting hydrophilic effect.

The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not considered to depart from the spirit and scope of the invention.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 5554739 [0016]
• US 5922118 [0016]

Claims (10)

A product comprising a bipolar plate for a fuel cell having an outer surface, with a cationic polymer layer or with a metal oxide layer over the outer surface, with a conductive and hydrophilic surface layer disposed over at least a portion of its outer region, wherein the conductive and hydrophilic surface layer comprises a conductive carbon material having a hydrophilic organic group, wherein the conductive carbon material is ionically attached to the cationic polymer layer or to the metal oxide layer. The product of claim 1, wherein the carbon materials are carbon black, graphite, carbon fiber, amorphous carbon or carbon nanotubes. The product of claim 2, wherein the surface layer is a microporous carbon layer and the carbon material is a carbon black. A product according to claim 1, wherein the hydrophilic organic group comprises at least one of a carboxylic acid, a benzenesulfonic acid, an alcohol, an amine, an amide, a polyethylene oxide, an ethoxyl, a methoxyl, a phosphonic acid, a naphthenesulfonic acid, a Salicylic acid, a phenylphosphonic acid, a benzoic acid, a phthalic acid group or a salt of any of the above-mentioned acids. The product of claim 1, wherein the surface layer has a water contact angle of 40 degrees or less and a contact resistance of less than about 20 mohm-cm ² at 200 psi compression pressure. The product of claim 1, wherein the bipolar plate comprises a graphite, carbon fiber or fiber reinforced carbon composite, and / or wherein the bipolar plate comprises a metal or metal alloy of aluminum, titanium, stainless steel, copper, nickel or chromium. A method comprising: providing a bipolar plate for a fuel cell comprising a carbon material disposed on at least a part of its outer region, bringing the carbon material into contact with an organic molecule having at least one hydrophilic group, and causing the organic molecule; reacting with the carbon material so that the hydrophilic group is covalently attached to the carbon material. The method of claim 7, wherein the bipolar plate comprises a graphite, carbon fiber, or fiber reinforced carbon composite, and / or wherein the carbon materials on the surface of the bipolar plate are formed by ionic adsorption, chemical vapor deposition, physical vapor deposition, atomic layer deposition. Spray coating, plasma deposition, dip coating, drain-down coating, printing, electrochemical coating or heat spraying, and / or wherein the carbon material is carbon black, graphite, amorphous carbon, carbon fiber or carbon nanotubes, and / or wherein the organic molecule is a diazonium salt represented by the chemical formula HG-Ar-N ₂ ⁺ X ^- , where HG is the hydrophilic group, Ar is an aromatic radical, -N ₂ ^{+ is} a diazonium radical, and X ^- is an anion, and / or wherein the diazonium salt is obtained by combining an aromat amine with the hydrophilic group with a nitrite, with nitrous acid, with nitrogen dioxide and / or nitric oxide in situ prepared. A method, comprising: providing a carbon material and a bipolar plate for a fuel cell, covalently attaching a hydrophilic organic group to the carbon material by a chemical reaction and depositing the carbon material on at least a portion of the exterior of the bipolar plate to form a conductive and hydrophilic surface layer , The method of claim 9, wherein the carbon material is a carbon black, graphite, amorphous carbon, carbon nanotube or carbon fiber, and / or wherein the carbon material is a carbon black particle, and / or wherein the chemical reaction is a reaction between an aromatic diazonium salt the hydrophilic organic group comprises, and the carbon material, and / or, wherein the carbon material is deposited on the bipolar plate by an ionic adsorption, by an electrochemical adsorption, by a coating or by a printing process, and / or the hydrophilic ones Group on the carbon material in an amount of about 0.10 to 5 mmol / m ² is present, and / or further comprising the assembly of a fuel cell stack using the bipolar plate.

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